Conventional car structures will be described by referring to FIGS. 1-5. In FIGS. 1--3, the numeral 1 designates a window formed in a side wall of a car body, and the numeral 2 designates a vertical structural member arranged in a vertical direction. The vertical structural members 2 are arranged on opposite sides of each of the windows 1 in corresponding positions. The numeral 3 designates a horizontal structural member. The horizontal structural members 3 are arranged above and below each of the windows 1 in corresponding positions, and are joined by welding, at their opposite ends, to the vertical structural members 1. The vertical and horizontal structural members 2 and 3 are both of Z-shaped cross section. The numeral 4 designates an outside plate that is joined to the exterior side of the vertical and horizontal structural members 2 and 3 by spot welding. The numeral 6 designates a weld produced by spot welding. The numeral 5 designates a window corner reinforcing member joined to one of four corners of each window 1 at which the vertical and horizontal structural members 2 and 3 are joined together. The window corner reinforcing member 5 is joined by welding to the vertical and horizontal structural members 2 and 3, so as to avoid stress concentration in each corner of the window 1, indicated as a section B in FIG. 3, at which the vertical and horizontal structural members 2 and 3 are joined together.
In the aforesaid construction, strain would be produced in the vertical structural member 2, horizontal structural member 3 and window corner reinforcing member 5 when they are joined together by welding, and the car body would not be acceptable in external appearance when completed. Even if an attempt were made to perform straightening, this would only result in increased cost because the operation would require a multiplicity of steps. Particularly, when the aforesaid members are formed of thin sheets, the strain produced would be great. Thus, the prior art construction shown and described hereinabove would suffer the disadvantage that it is not possible to obtain a car structure that, overall, is light in weight.
Meanwhile, in sections C and D (FIG. 3), which each constitute a joint between the vertical structural member 2, horizontal structural member 3 and window corner reinforcing member 5, stress concentration would occur at end portions of each weld. Thus, the strength of each joint between the members 2, 3 and 5 would be necessary to increase the thickness of the members 2, 3 and 5 if it is desired to increase the strength of the joints, thereby making it impossible to lessen the weight of the car body. Also, a stress, .sigma..sub.m2, of a very high magnitude would act on a portion of each outside plate 4 corresponding in position to the window corner reinforcing member 5 due to the curvature of the window corners, as shown in FIG. 10 by a curve e. Thus, the outside plates 4 should be able to withstand the aforesaid stress of high magnitude. This has made it necessary to increase the strength of the outside plates 4, as by increasing the thickness, thereby making it impossible to obtain an overall light weight car body structure.
Other constructions of a car structure of the prior art will be described by referring to FIGS. 4 and 5. In FIG. 4, the vertical structural member 2 and horizontal structural member 3 are joined together with a flat plate joint 7.
In this construction, when an out-of-plane bending force acts on a side wall portion having this car structure, the flat plate joint 7 would merely bear such bending force with flexural rigidity, so that the structure could not be expected to have high rigidity. When a shearing force acts between the vertical structural member 2 and horizontal structural member 3, such shearing force would be borne by the flat plate joint 7 alone, so that the structure could not be expected to have high rigidity.
In the construction shown in FIG. 5, the vertical structural member 2 and horizontal structural member 3 are joined together with a joint 8 having a flange 9 adapted to come into contact with an inner flange of the vertical structural member 2, and a flange 10 adapted to come into contact with a web of the vertical structural member 2. The joint 8 is constructed such that adjacent planar surface portions are at right angles to each other.
In the aforesaid construction, when an out-of-plane bending force acts on the joint between the vertical structural member 2 and horizontal structural member 3, such force would be borne by the bending rigidity of the flanges 9 and 10 alone, so that the structure could not be expected to have high rigidity. When an in-face shearing force acts on the joint between the vertical structural member 2 and horizontal structural member 3, the use of the joint 8 would suffer the disadvantage that the structure could not be expected to have sufficiently high rigidity. This would make it necessary to increase the thicknesses of the members 2, 3 and 4 to obtain the desired rigidity, making it impossible to obtain a car body having an overall light weight. If an additional reinforcing member were used to increase the strength of the structure by connecting it to the joint between the vertical structural member 2 and horizontal structural member 3, the attempt would merely increase the weight of the structure.